How could WASI solve (or be involved in solving) this problem?

I suspect confusion with the WebAssembly Component Model — whose development is somewhat intertwined with that of WASI's.

It defines a function call ABI between sandboxes. No object is in shared memory: parameters are passed by value or by handle. Has its own IDL and ABI that languages' ABIs need to have adaptors to, if they don't conform.